The present invention relates to a speech signal coding and decoding systems and, more particularly, to a highly efficient speech signal coding and decoding system which transmits compressed speech signals in the form of a plurality of speech parameters and G3 facsimile signals by time divisional transmission over the same transmission line.
With recent spread of exclusive digital data transmission lines, there are prosperous attempts by enterprises to construct exclusive internal digital networks, and the speech signal coding and decoding systems with lower bit rates has been progressed from the standpoint of the economy of digital data transmission lines. However, to reduce the bit rate in the coding of speech signals without quality sacrifice therein, it is necessary to code the speech signals by taking features thereof into considerations, causing problems in the proper transmission of facsimile signals, which are non-speech signals. In other words, it is difficult to construct a network without taking telephone/facsimile features into considerations.
U.S. Pat. No. 4,876,696 discloses a technique for solving the above problem. In the disclosed technique, the at coding side provided are a speech signal encoder, and a G3 facsimile signal demodulator for converting G3 facsimile signal into digital data. Thus, when the G3 facsimile signals is input, the input signals are converted by the G3 facsimile signal demodulator into the digital data and transmitted. Similarly, at the decoding side provided are a speech signal decoder, and a G3 facsimile signal modulator. When G3 facsimile data is received as digital data, it is converted by the G3 facsimile modulator into a modem signal. In this way, the transmission of G3 facsimile signals is realized.
FIG. 4 shows data frame formats in this technique. The frame of speech data comprises a frame sync signal S provided at the start and following speech signal parameter data obtained as a result of highly efficient coding of speech signal. For G3 facsimile data, there are two different frames, i.e., a low speed frame and a high speed frame. The low speed frame comprises a head frame sync signal provided at the start, then G3 facsimile frame identification data indicative of the G3 facsimile data frame, and then G3 facsimile low speed data. Likewise, the high speed frame comprises a head frame sync signal, G3 facsimile identification data, and G3 facsimile high speed data.
In this prior art system, speech signals is divided into a plurality of frames each having a constant time interval and transmitted by frame-by-frame transmission. Therefore, it is necessary to provide an overhead in the frame, the overhead including a frame sync bit for establishing the frame synchronization, identification information representing whether the frame data is speed parameter data or G3 facsimile data, kind of G3 facsimile data (i.e., either CCITT V21 or V29/V27 ter) and information indicative of the number of data pieces. That is, it is inevitable to transmit G3 facsimile data in the frame duration without the overhead. This means that 9,600 bps facsimile high speed data, for instance, can not be transmitted over a 9,600 bps line. By way of example, in case of coding speech signals with 16 kbps to produce 20-msec speech data frames, each frame has 320 bits. On the other hand, for facsimile data transmitted in 20-msec frames the number of bits per frame is 192. When using the speech data frame configuration noted above, 320-192, i.e., 128, bits are residual bits. These residual bits are assigned to the above head. However, when coding speech signal with 9.6 kbps for transmission with a frame duration of 20 msec., the number of bits per frame is 192, and the above residual bits can not be secured for assignment to the overhead. This causes the above problems.